The present invention relates to the measurement of corrugated board characteristics and more particularly to an apparatus and method for measuring multiple corrugated board characteristics using laser based sensors.
Corrugated board is formed by bonding, with a starch-based, aqueous glue, one or more layers of paper, called liner, to the outside of one or more fluted paper layers, called medium. The resulting corrugated board is then converted into boxes for the packaging of commercial and industrial products. A typical converting sequence involves printing on the board's external surface using a sheet-fed flexographic printing press, followed by die-cutting to provide the appropriate flaps and slots, and finally, by folding and gluing to form the finished box.
To protect packaged goods from external loads, such as those generated by stacking laden boxes on top of one another, board strength should be maximized for a given board weight. A key predictor of box strength is the board's caliper, which influences its mechanical stiffness. Board caliper also influences print quality, which is affected by the dimensional interference between the printing blanket and the board's surface. Variables such as the flute height, liner thickness, glue application rate, and mechanical loads, are set to produce an application-specific board caliper. Therefore, improper set-up or control can result in a final board that is too thick or thin for its intended purpose. On-line measurement of board caliper will therefore improve board quality and reduce scrap, by more promptly identifying improper operating conditions.
During manufacture the board may be improperly formed, resulting in common, undesirable board flaws such as delaminated regions and crushed or collapsed regions, which typically occur near the board's outer edges (within about 3 inches of the outer edge). Delaminations are the result of inadequate bonding between the board's layers, and may be traced to multiple sources, such as insufficient glue volume, improper temperature during the application of the glue, and insufficient or excessive drying. Board crushing also has multiple causes, such as improperly formed or skipped flutes (which are often caused by misaligned fluting rolls, and are more likely to occur as the speed is increased), as well as contact with contaminants, such as pieces of scrap paper, which when stuck to the surface of rolls or belts comes into loaded contact with the board.
Flawed boards are usually scrapped prior to or during converting, or returned to the board manufacturer for credit, both of which reduce the profitability of the corrugating operation. On-line detection and diagnosis of board flaws will therefore reduce both in-plant scrap and shipment of substandard product, by permitting more prompt problem recognition and resolution.
During manufacture the medium's flute tips are forced against the inside surface of the liner to ensure adequate bonding between the two layers. This contact pressure along the glue-line interface between the two layers causes a slight deformation of the liner, generating a visible pattern of raised lines, which is sometimes referred to as a "wash-board" pattern. These undulating flute-ridges may result in a printed image of variable intensity, and, particularly when glossy inks are used, may appear as a series of shiny ridges that obscure the printed image. In addition to bonding loads, there are other process variables that affect the degree of flute-ridging, such as moisture levels which cause paper shrinkage during drying, and sheet tension, which when increased tends to stretch the liner to reduce ridging.
Use of corrugated board for printed consumer packaging is increasing, so more emphasis is being placed on surface flatness. Measuring the amplitude of flute-ridges (defined here as the peak-to-valley value, or the distance from the peak of a flute ridge to the lowest point between it and an adjacent flute ridge) will allow corrugators to grade the board's printability and cull unacceptable product. On-line measurement will also allow corrugators to determine how flute-ridging is affected by complex process changes, perhaps facilitating its closed-loop control.
On-line measurement of caliper and surface roughness, as well as detection of flaws, has been successfully performed in connection with several known sheet-producing applications. For example, caliper measurement, using both contacting and non-contacting means, has been applied to the production of paper since the early 1970's. Flaw detection on papermaking machines has also been successfully used for more than a decade. However, while existing technologies for measuring caliper and surface roughness of paper being manufactured on papermaking machines and for detecting flaws in such paper, could in theory be applied to a corrugating operation, the aggregate cost of a comprehensive solution incorporating existing systems would be prohibitive. Each of the commercially available systems make these measurements through the use of a different sensor technology for each measurement (such as magnetic reluctance for caliper, optical scattering for roughness, and CCD-based imaging for flaws), and each sensor is relatively complex and expensive.
The principle of laser triangulation has also been applied to the scanning measurement of caliper in sheet manufacturing processes, including in papermaking machines. U.S. Pat. No. 5,210,593 describes a scanning, laser-based measurement apparatus that employs the principle of laser triangulation to measure the caliper of a paper web from point to point across its width.
Corrugating plants are traditionally lower margin operations than paper mills, and for this reason, operators of corrugating plants are less able to afford scanning measurement devices. In addition, corrugators are far narrower (typical width is 87 inches) than papermaking machines (whose widths may exceed 400 inches), so their shorter, stiffer cross-machine elements (rolls, drying plates, etc.) do not cause the localized caliper changes that a scanning apparatus is designed to measure. These factors may explain why scanning, laser-based, caliper measurement devices have not been used on corrugators.
It is therefore a principal object of present invention to provide an apparatus and method for on-line measurement of a number of corrugated board characteristics.
It is the further object of the present invention to provide an apparatus and method for on-line measurement of corrugated board characteristics which utilizes a laser-based sensor which has the capability of measuring more than one characteristic.